Maybe it will be fixed in a future patch, but I don't get my hopes up too much because I read somewhere that this issue has been around since the Beta.
Since I don't have a spare 1080p display, I tried disabling Scaling in the nVidia Control Panel, in other to prevent the GPU from automatically upscaling the image to 1440p. It works fine with most other games, but for some reason it doesn't work with PoE: the game is upscaled anyways (